Abstract: OBJECTIVE: Randomized trials have established the effectiveness of 5-fluorouracil-based adjuvant chemotherapy for stage III resectable colon cancer but the toxicity has not been well established outside the trial setting. The objective of this study was to estimate the risk of various toxicity-related endpoints among the elderly patients. METHODS: Patients diagnosed with stage III colon cancer in 1991 to 2005 were identified from the Surveillance, Epidemiology, and End Results-Medicare database. Chemotherapy use within 3 months after tumor resection was identified from submitted claims. We reported the 3-month cumulative incidence rate (CIR) for gastrointestinal and hematologic toxicities. The risk of ischemic heart disease in relation to chemotherapy use and length was assessed using Cox regression models, stratified by age and comorbidity subgroups. RESULTS: Of the 12,099 patients, 63.9% (n=7740) received adjuvant chemotherapy. Common gastrointestinal and hematologic toxicities among chemotherapy group include volume depletion disorder (CIR=9.1%), agranulocytosis (CIR=3.4%), diarrhea (CIR=2.4%), nausea and vomiting (CIR=2.3%). Chemotherapy use was significantly associated with the onset of these toxicities [hazard ratio (HR)=2.76; 95% confidence interval (95% CI)=2.42-3.15]. The risk of ischemic heart disease was slightly associated with chemotherapy use (HR=1.08, 95% CI=0.96-1.22), but significant only among patients aged <75 years with a comorbidity score equal to 0 (HR=1.70; 95% CI=1.02-2.84). CONCLUSIONS: Chemotherapy was associated with the increased risk of developing gastrointestinal, hematologic, and cardiac toxicities in patients with colon cancer. On account of the effects of these side effects on treatment discontinuation, rehospitalization, and overall health status, some close monitoring and preventive measures may be emphasized to maximize the benefits of adjuvant chemotherapy.